25 August 2013

Scientists will use cosmic rays to peer inside Fukushima reactor

The news just keeps coming from Fukushima, questioning both the reported volume and radioactivity of the water leaking into the ocean, and projections continue to get less optimistic by the day. The only encouraging part is that the potential solutions seem to be almost as numerous as the newly reported problems, as authorities are in the midst of building a steel wall to contain flowing ground water, and now want to freeze a section of the perimeter to create a second wall of icy earth. These are all ultimately stop-gap measures, however, designed to minimize the continuing impact while cleanup efforts continue in earnest. The biggest problem for actually improving the state of the core: cleanup crews know very little about what’s actually going on inside.

More than six feet of concrete and eight inches of steel surround the core of the reactors, so getting a picture of the situation inside has proved extremely difficult. Radiation levels are obviously too high for human workers, and even pose problems for most potential robotic solutions. The international community needed to figure out how to see inside the core without actually going inside, either personally or remotely, and that meant turning to those few high-energy particles that could penetrate its thick shielding twice, enter on one side and emerge on the other. After many months of campaigning, scientists at Los Alamos National Laboratories are proceeding with a plan to take advantage of one such particle: muons originating from cosmic rays.
A simple schematic representation of the detector.

In essence, the idea is similar to many conventional forms of imaging, tracking the path of rays to walk back to the material that must have caused any observed deflections. The novel part of the plan is the type and source of the particle; muons have such high energy that even a detonating nuclear bomb won’t create them. One readily available source of high-energy muons is collisions between cosmic protons and atoms in the upper atmosphere. The pions created in these events quickly decay into muons that are so penetrative we can detect them at the Soudan II detector more than 700 meters below ground.

The plan uses detectors adapted from border security machines designed for finding weapons-grade plutonium in cargo containers. Two detectors will be used, one to measure the original trajectory before entry, and another to measure the final trajectory after exit. They’ve already tested the idea on a small-scale mockup of the reactor and on a working power plant at the University of New Mexico, so they’re confident in their technique. Researcher Haruo Miyadera says the plan won’t produce much in the way of scientific surprises, adding that “all the challenges are in engineering, not in science.”

The technique is most useful with regard to heavy materials, and can tell the researchers the locations of large chunks of fallen building material, as well as the fate of the reactor’s nuclear fuel, much of which has melted into a concrete vessel below the core. Water irradiated by this fuel has been leaking to various extents since shortly after the earthquake of March, 2011.

This test saw the hole in this lead construct through six meters of concrete, and was conducted in just over a week; Fukushima Daiichi is much larger, and the detector will run for months.

Since the source of the “interrogator” particles is the random collision of particles with the atmosphere, it will take a long time to resolve a clear picture of what’s going on inside. Each muon is a data point, but we can’t control their frequency or trajectory and must wait patiently for enough useful hits to accrue. The Japanese government hopes to have the detectors set up in 2015, but doesn’t expect to have a useful picture for several months after that.

At present, Fukushima Daiichi is in a state that could be called stable but dangerous. Japanese authorities are working to contain its only current method of damaging the surrounding area, the leaks due to cracks in containment. It’s unlikely that the situation will take a dramatic turn for the worse at this point, and the team from Los Alamos labs is hopeful its detector can help start a safe clearing of core debris by 2020. After that, they say, the detector could become part of the regular nuclear cleanup and maintenance regime.

Despite being involved in cleaning up this messy situation, LANL scientist Christopher Morris remains optimistic about nuclear power as a whole. He reminded NBC News that nobody has yet died as a direct result of the radioactivity at the Fukushima plant.

Now read: 3D graphene nanoballs could make supercapacitors more super.

Research paper: doi.org/10.1063/1.4808210 – “Imaging Fukushima Daiichi reactors with muons”


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